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Repeated eastward shifting of subduction-related magmatism between Jurassic and Present generated a collage of four magmatic arcs in North Chile (21-26 S). Focussing on the Late Cretaceous-Eocene arc, this study explores the evolution of geochemical and isotopic signatures of arc mag- matic rocks through time. Temporal geochemical variations include systematic steepening of heavy rare earth element patterns and increasing Sr and Al contents with decreasing age. Gradually increasing La/Yb and Sr/Y ratios between 78 Ma and 39 Ma appear to monitor gradual crustal thick- ening by newly underplated basaltic crust, whereas sharply rising La/Yb and Sr/Y ratios in late Eocene syn/post-tectonic granites and granodiorites reflect Incaic (38.5 Ma) shorten- ing and thickening of arc crust to 45 km, consistent with regional tectonic field constraints. Support for the timing and magnitude of crustal thickening comes from similarities with chemical signatures from the modern southern volcanic zone (SVZ), from trace element modeling and from regional geologic constraints. On the scale studied, the geochemi- cal variations presented correspond to a temporal transition from common Phanerozoic island-arc type to Archean type high-Al trondhjemite-tonalite-dacite (TTD) and granodiorite (TTG) crust. Comparisons with adjacent older and younger Andean magmatic arcs in this region show similar geochemi- cal patterns; the thicker the crust the more high-Al TTD/TTG like the chemical signatures are. On the larger scale of the Andean orogeny, our results support earlier models of Archean crustal growth which suggest that large portions of early Earth's high-Al TTD/TTG type continental crust may have been generated by recycling mafic underplate of thick orogenic crust.

Evolutionary geochemical patterns of Late Cretaceous to Eocene arc magmatic rocks in North Chile: implications for Archean crustal growth